Муравьи Camponotus schmitzi гнездятся эксклюзивно и только в полых усиках-стебельках кувшинчиков N. bicalcarata (Burbidge
1880; Clarke
and Kitching 1995). They feed on nectar secreted from the
peristome and on prey items captured by their host plant,
which they haul out of the pitcher fluid (Clarke and
Kitching 1995; Merbach et al. 1999). In striking contrast to
other ants which are regularly captured and digested in the
pitchers of N. bicalcarata, C. schmitzi ants are able to run
safely across the wet and slippery pitcher mouth (peristome)
and never become trapped (Clarke and Kitching
1995). Moreover, they forage for food inside the pitchers.
Highly unusually for ants, they are able to dive and swim in
the pool of digestive pitcher fluid. On their foraging trips,
they repeatedly enter and leave the fluid, apparently unaffected
by surface tension forces.

Плавание по поверхности воды как адаптация от случайного падения в воду или для crossing small water bodies has been
reported for several ant genera, including Camponotus
(DuBois and Jander 1985). While surface-swimming ants
generate thrust only by moving their front legs, with the
middle and hind legs acting as a rudder at the water surface,
the swimming behavior of C. schmitzi ants is different
in that it involves movements of all legs under water.
Here we report field and laboratory observations on the
aquatic foraging behavior of C. schmitzi and present a
kinematic analysis of swimming locomotion in comparison
with normal (terrestrial) running. We investigate (1) how
gait patterns for swimming and running are related to each
other and whether a different locomotor program is used
for swimming, (2) whether swimming is a special adaptation
of C. schmitzi and (3) how swimming ants generate
thrust.

Исследователи обнаружили, что рабочие муравьи C. schmitzi плавают with a stereotyped
tripod pattern similar to that used by many insects for
running on land (Hughes 1952). However, there were
characteristic differences between the leg kinematics of
swimming and running in C. schmitzi ants. C. schmitzi ants
vary velocity and leg extension between power and return
stroke to generate thrust. During the power stroke, the front
and middle legs were extended and moved rapidly whereas
in the return stroke they moved slower and were held closer
to the body. Similar asymmetries between power and
return strokes are widespread among swimming animals
(Alexander 1982). The hind legs contributed only little to
the overall thrust, suggesting that they mainly serve for
stabilization and steering.

Кинематические различия между плаванием и управлением, вероятно, основаны на различных моторных программах ног. Это может частично отражать
ограничения различных физических сред.
During running, the air allows
faster leg protraction than the denser water, but the ground
contact constrains the extension of the legs and limits the
duty factor in comparison to swimming. Legged locomotion
on land often requires that the insect’s center of gravity
lies within the tripod of the three legs on the ground (Ting
et al. 1994). Static stability in the absence of foot adhesion
requires that legs are on the ground for more than half the
time, corresponding to a duty factor of at least 0.5. This
constraint is relaxed during swimming, where the insect is
supported by water, making smaller duty factors possible.

Кроме возможного прямого влияния физических ограничений,
the leg movements are modified for running and
swimming in a segment-specific way, leading to characteristic
differences in leg orientation, leg extension and
duty factor. This indicates that there may indeed be different
pattern generators for the different joints of each ant
leg, which possess some flexibility and independence,
consistent with previous findings on the locomotion and
pattern generation in stick insects (BaЁssler and BuЁschges
1998).

Несмотря на the segment-specific modifications for running
and swimming, C. schmitzi ants produce a tripod-like
coordination between the legs for both types of locomotion.
The motor programme for each individual leg depends on
the interaction of central and coordinating influences, as
well as peripheral sensory input (providing information
about joint positions, movements and loads) (Graham
1985; BaЁssler and BuЁschges 1998; Cruse et al. 2009). The
sensory input for legs moving freely in water is obviously
very different from that experienced by legs during running.
This different sensory input may explain why tethered
C. rufifemur ants did not show any coordinated leg
movements underwater. The disposition and neuronal
ability of C. schmitzi ants to perform coordinated swimming
movements without any substrate contact thus
appears to be an adaptation to the ants’ amphibious lifestyle.
Our experiments also demonstrate that the swimming
movements in C. schmitzi can be triggered by submersion.
It is unclear whether the ants react to the submersion
itself (using mechanical, optical or physiological cues) or
whether the higher drag forces acting on the legs provide
the necessary sensory input to convert the erratic leg
movements performed in air to the coordinated gait
underwater.

Некоторые другие членистоногие показывают различные способы ломоции в/на воде и по земле.
The patterns of leg
coordination are in most cases different between aquatic/
neustic and terrestrial locomotion [например, водные жуки
(Hughes 1958), клопы (Bowdan 1978; Wendler et al.
1985), прямокрылые (Pfluger and Burrows 1978), пауки
(Barnes and Barth 1991)]. In some cases, intermediate
types of locomotion were found (such as intermediates
between walking and rowing in semi-aquatic spiders;
Barnes and Barth 1991), again suggesting that the different
movement patterns are produced by a flexible system
capable of generating variable patterns of motor output.

Плавательное поведение что авторы наблюдали у муравья C. schmitzi has
several similarities to the surface-swimming behavior of
other ants, which was described in detail for Camponotus
americanus by DuBois and Jander (1985). First, most of
the thrust is produced by the front legs while the hind legs
move relatively little and are held in a backwards orientation.
Second, the front legs are extended during the power
stroke and flexed during the return stroke. In contrast to
C. americanus, however, C. schmitzi keeps all its legs
underwater continuously and both middle and hind legs
contribute to swimming by performing rhythmic motions,
although their movements are smaller than those of the
front legs. Moreover, the swimming movements of
C. schmitzi appear to be much faster (step frequency
9.35 Hz in C. schmitzi vs. 1.32 Hz in C. americanus,
DuBois and Jander 1985).

Плавание это сравнительно редкое поведение среди муравьёв,
обычно демонстрируемое только отдельными рабочими особями случайно упавшими в воду or when they actively cross
small bodies of water (DuBois and Jander 1985). Ant
species differ in their readiness to perform such surfaceswimming
behavior when dropped into water (W.F.,
personal observation). One exception in which swimming
is part of the normal behavioral repertoire is the specialized
mangrove-inhabiting species Polyrhachis sokolova
(Nielsen 1997; Attenborough 2005). In all these ants,
however, regardless of rarity of the behavior, the hind
legs are not submerged during swimming, but remain
dewetted at the surface of the water. This, taken together
with the 79 greater step frequency in C. schmitzi, indicates
a greater degree of specialization for its amphibious
lifestyle.

Таксономия крупного подрода Camponotus-subgenus
Colobopsis остаётся неразработанной (Brady et al. 2000) и ни одного близкого родственника у C. schmitzi до сих пор не найдено.
Однако, уникальность этой ассоциации между родами Nepenthes и Camponotus показывает, что это сравнительно недавнее эволюционное
достижение. This is consistent
with our finding that C. schmitzi uses a tripod
coordination for swimming, indicating a relatively simple
adaptation which may have required short evolutionary
periods. It appears that locomotion in water with a coordination
between the legs similar to walking and running
is common among terrestrial insects that only occasionally
swim (Shumakova et al. 2003), whereas many truly
aquatic insects have evolved more distinct ‘‘swimming
gaits’’ (see overview in Shumakova et al. 2003). It seems
plausible that the ancestors of C. schmitzi were able to
perform a surface-swimming behavior like C. americanus
and that further modifications were brought about by a
change in the surface chemistry of the cuticle, making the
legs more hydrophilic and allowing them to become submerged.
Such a change in surface chemistry would also
make it easier for C. schmitzi to enter the digestive fluid
when starting to forage inside a pitcher. Indeed, we have
preliminary evidence that the cuticle of C. schmitzi is
more wettable than that of other Camponotus ants (D.G.T.,
in preparation).